1. Field of the Invention
The present invention relates to a slip control system for an automotive vehicle and, more particularly, to a slip control system for an automotive vehicle so adapted as to suppress a slip or spin of a wheel against a road surface in accordance with the driving state of the automotive vehicle body.
2. Description of Related Art
Heretofore, as disclosed in Japanese Patent Laid-open (kokai) Publication No. 251,268/1987, there is known a slip control system having a traction control function for suppressing a slip or spin of the driven wheel by recovering a grip of the driven wheel by reducing output of the engine as well as by braking the driven wheel when a slip value of the driven wheel against a road surface reaches a predetermined value or higher during accelerating the automotive vehicle body.
Japanese Patent Laid-open (kokai) Publication No. 255,558/1985 discloses a slip control system (an ABS control unit) equipped with an anti-skid control function for controlling a braking pressure to be so applied to each of the wheels as to provide an appropriate braking force by preventing the wheels from being locked during operation of the brakes.
In executing control over a slip or spin of the driven wheel of the automotive vehicle on a road surface, the control state varies to a great extent with a road surface friction coefficient .mu.. For instance, when the automotive vehicle runs on a road having a small road surface friction coefficient .mu., such as a frozen road, a friction resistance of the wheel acting upon such a road during driving the automotive vehicle on the road is extremely small so that the behavior of the wheel varies to a great extent during driving on the road than during driving on ordinary road, when the wheel is braked by the slip control system.
It is desired, therefore, that the control state of the wheel by the slip control system is varied with a road surface friction coefficient .mu.. However, it is impossible to determine an accurate road surface friction coefficient .mu. at the time of start of slip control so that the slip control cannot be performed in an appropriate way so as to correspond to the state of a road surface. In other words, the road surface friction coefficient .mu. is usually calculated in accordance with a slip value of the driven wheel changing by executing the slip control so that no accurate road surface friction coefficient .mu. at the time of start of slip control can be determined. Therefore, conventional slip control system is so arranged as to determine the timing of start of the slip control in accordance with an initial value of an estimated road surface friction coefficient .mu. to be employed at the time of start of slip control, an initial value being set to a maximum value. Hence, the timing of starting the slip control cannot be determined in an accurate way during driving on a road having a low surface temperature, so that a response to the control is made poor.
In order to improve the response to the control during driving the automotive vehicle on a road having a low road surface temperature, it is considered to set an initial value of an estimated road surface friction coefficient .mu. to a small value corresponding to the low road surface friction coefficient .mu.. In this case, however, the problem may arise that the behavior of the automotive vehicle becomes unstable as a result of frequent execution of slip control during driving the automotive vehicle on ordinary road other than a frozen road or the like.